An unvented cylinder is fine for DHW, but unless you put a buffer on the heating side you get poor control over the radiators which are constantly heating up/cooling off as the thermostat cuts in and out. Also boiler cycling. With a thermal store you can keep the rads at an even temperature with TRVs. It is a much more comfortable option IMHO. The downside is that the tank needs to be twice the size of an unvented, depending on boiler size.
